1 Technical Field of the Invention
The present invention relates generally to an improved structure of a moisture sensor designed to measure a moisture content of a specified gas electrochemically and a fuel cell system using such a moisture sensor.
2 Background Art
Solid polymer fuel cells usually need to be humidified in order to keep the conductivity of an electrolyte film in the fuel cell. When a moisture content of the fuel cell is small, so that the electrolyte film is dry, it will cause an inner resistance of the fuel cell to be increased, thus resulting in a decrease in output voltage. Alternatively, when the moisture content of the fuel cell is too great, it will cause electrodes of the fuel cell to be covered with water, which disturbs diffusion of reactants: oxygen and hydrogen, resulting in a decrease in output voltage.
Thus, increasing the operating efficiency of the fuel cells requires fine control of a moisture content of the fuel cells precisely. For example, Japanese Patent First Publication No. 2001-236977 proposes a control system which measures the humidity of unreacted fuel cell exhaust gas using a humidity sensor and controls the amount of moisture added to the electrolyte film as a function of an output of the humidity sensor.
The humidity sensor, as employed in the above system, is of a capacitance type which is slow in response. Additionally, such a capacitance type humidity sensor encounters a difficulty in measuring the quantity of moisture correctly when the humidity is more than 100%, that is, when there are waterdrops.